1. Field of the Invention
The present invention relates to a process for the ammoxidation of light hydrocarbons, preferably olefins, to unsaturated nitriles, such as acrylonitrile.
2. Description of the Prior Art
It is well known that olefins can be oxidized to oxgenated hydrocarbons, such as unsaturated aldehydes and acids, for example acrolein and methacrolein, and acrylic and methacrylic acid. It is also well known that olefins can be ammoxidized to unsaturated nitriles such as acrylonitrile and methacrylonitrile. The value of these products is generally well recognized, with acrylonitrile being among the most valuable monomers available for producing useful polymeric products.
Many catalyst systems have been developed which are useful for oxidation and ammoxidation of light hydrocarbons, with special attention being given to the ammoxidation of propylene to form acrylonitrile.
One exceptionally active catalyst for oxidation/ammoxidation is the catalyst disclosed in my U.S. Pat. No. 4,018,712, the teachings of which are incorporated herein by reference. This catalyst contains the elements antimony, uranium, iron, bismuth, and molybdenum, and optionally, nickel or cobalt in a catalytically active oxidized state. This catalyst is exceptionally active and very suitable for the ammoxidation of propylene to acrylonitrile.
Other catalyst systems have been developed composed of oxides of molybdeum and tellurium, as described in U.S. Pat. No. 3,164,626, the teachings of which are incorporated herein by reference. A somewhat related catalyst for the ammoxidation of olefins composed of the oxides of tellurium and cerium is described in U.S. Pat. No. 3,446,834, the teachings of which are incorporated herein by reference.
It is also known to use in the ammoxidation process catalysts composed of oxides of molybdenum, tellurium, manganese, and phosphorus as described in U.S. Pat. No. 3,335,169, the teachings of which are incorporated herein by reference.
Another suitable catalyst for the ammoxidation process is composed of oxides of iron, antimony, vanadium, molybdenum, tungsten, and tellurium, as described in U.S. Pat. No. 3,668,147, the teachings of which are incorporated herein by reference.
Despite the great number of catalysts known to be useful for the oxidation/ammoxidation reactions, there is a continued need in the art to develop catalysts with even greater activity, and which increase the yield of desired products while minimizing yields of undesired products. In a relatively highly developed process, such as production of acrylonitrile by the ammoxidation of propylene, a very small yield increase, even on the order of 1/2 or 1%, represents a very significant commercial advance since the volume of acrylonitrile on a world-wide basis is extremely large.
It has been discovered that as catalyst activity increases, there are some attending undesirable consequences. The exact nature of the problem is difficult to fully understand based on present knowledge of what precisely occurs in the oxidation/ammoxidation process. It is believed that some undesirable gas phase reactions occur even in a well fluidized bed. This problem of side reactions seems to be worse with more active catalysts.
Another consequence of the use of more active catalysts is that as the activity of the catalyst has improved, the amount of feed and throughput increase, and heat generation due to the exothermic nature of the reactions, increases. Normally, plant facilities for making acrylonitrile cannot be extensively modified to but accommodate new and more active catalyst because of the large scale economies involved. Therefore, one must make do with the existing equipment with only minor modifications thereto. It is believed that the very enhanced activity of such catalysts, including the very active catalyst disclosed in the U.S. Pat. No. 4,018,712, referred to above presents a hindrance to optimum commercial operation in a process using such catalyst.
It has been discovered, quite unexpectedly, that addition of an inert material, such as alumina to dilute the catalyst significantly increases the yield of desired products, while minimizing in an acceptable manner the yield of undesirable by-products.
Accordingly, the present invention provides a process for oxidation/ammoxidation comprising charging to a fluidized bed oxidation/ammoxidation reaction zone containing a mixture of active oxidation/ammoxidation catalyst and particles of a relatively inert material having a particle size distribution compatible with fluidization in said reaction zone, and withdrawing from said reaction zone an oxidation/ammoxidation product.
In another embodiment, the present invention provides a process for the ammoxidation of propylene to acrylonitrile comprising charging to a fluidized bed ammoxidation reaction zone operated at ammoxidation conditions a feed comprising propylene, ammonia and oxygen, and wherein said ammoxidation reaction zone contains a fluidized bed mixture of active ammoxidation catalyst and particles of a relatively inert material having a particle size distribution compatible with fluidization in said ammoxidation reaction zone, and wherein acrylonitrile is withdrawn from said reaction zone as a produce of the process.
In a more specific embodiment, the present invention provides a process for the ammoxidation of propylene to acrylonitrile. The process involves feeding propylene, ammonia, and oxygen to a fluidized bed reaction zone. The resulting product, acrylonitrile, is recovered from the reaction zone. The fluidized bed in the reaction zone comprises a highly active ammoxidation catalyst comprised of antimony, uranium, and iron along with possibly other metals such as bismuth and molybdenum, preferably on a support. Where a support is used, the catalyst comprises at least 5% up to about 90%, preferably 10 to 50%, by weight of the catalytic composite material. Any known support materials can be used, such as, silica, alumina, zirconia, alundum, silicon carbide, aluminasilica, and the inorganic phosphates, silicates, aluminates, borates, and carbonates which are stable under the reaction conditions in the feed reaction zone and do not significantly reduce the catalytic activity of the active portion of the catalyst.
The composite catalyst is diluted with an inert substance which remains in the solid state in the reaction zone during the ammoxidation reaction. The resulting fluidized bed contains 2 to 50%, preferably 5 to 40% of the inert discrete particulate substances which has a particle size distribution compatible with fluidization. The inert substance can be silica, alumina, zirconia, alundum, silicon carbide, aluminasilica, and the inorganic phosphates, silicates, aluminates, borates, and carbonates which are stable under the reaction condition employed. Alpha-alumina is preferred. As a result, acrylonitrile in greater yields and by-products in lesser yields are produced as compared to the use of the same active ammoxidation catalyst not in association with inert discrete particles.
The preferred active ammoxidation catalyst has the following empirical formula EQU Sb.sub.a U.sub.b Fe.sub.c Bi.sub.d Mo.sub.e Me.sub.f O.sub.g
Me is nickel or cobalt, a is 1-10, b is 0.1 to 5, c is 0.1 to 5, d is 0.001 to 0.1, e is 0.001 to 0.1, f is 0 to 0.1, and g is a number taken to satisfy the valences of the quantities of Sb, U, Fe, Bi, and Mo, including Ni and Co, if present, in the oxidation state in which they exist in the catalyst.